1. Field of the Invention
The present invention relates to a printing apparatus adapted to eject ink droplets from ink ducts, comprising at least one ink duct provided with an electromechanical transducer, a drive circuit provided with a pulse generator to energize the transducer, a measuring circuit for measuring an electrical signal generated by the transducer in response to the energization, and means to break the circuits in such manner that the drive circuit is open if the measuring circuit is closed.
2. Background Art
A printing apparatus of this kind is known from U.S. Pat. No. 4,498,088. In this printing apparatus, which is of the xe2x80x9cdrop-on-demandxe2x80x9d type, the drive circuit applies an electrical pulse across the electromechanical transducer, more particularly a piezo element, so that the transducer is energized and generates a pressure wave in the ink duct. An ink droplet is ejected from the ink duct as a result. To guarantee reliability of such a printing apparatus, means are provided to detect breakdown of the ink duct, e.g. due to the presence of an air bubble in said duct. These means form part of a measuring system and comprise a measuring circuit with which it is possible to measure the resulting vibration in the ink duct after a pressure wave has been generated by the transducer. For this purpose, the transducer is used as a sensor: Thus, a vibration in the duct in turn results in the deformation of the electromechanical transducer, so that it generates an electrical signal. If air bubbles are present in the duct, this results in another vibration and consequently another electrical signal. A breakdown of an ink duct can thus be readily detected by measuring the electrical signal. A repair operation for the duct in question can then be carried out. One important disadvantage of a printing apparatus of this kind is that in order to check the condition of the ink ducts, the printing apparatus must leave the normal printing mode, i.e. the mode in which at least one ink duct ejects ink droplets for generating an image on a substrate, to pass to a measuring mode. In the measuring mode the transducer is energized so that the ink duct is vibrated but it is not possible to achieve ejection of an ink droplet from that duct. The resulting electrical signal is measured, and after this it is possible to determine whether there are any air bubbles in the ink duct. After the ink duct has been checked, the printing apparatus is returned to the printing mode, possibly after a repair operation has been carried out. The need to switch between a printing mode and a measuring mode results in a loss of productivity of the printing apparatus. Productivity will further fall with increasing reliability requirements for the printing apparatus, which means that the interval of time between the measuring modes has to be reduced. In addition to loss of productivity, the known printing apparatus has the disadvantage that two drive circuits provided with pulse generators are required for the transducer: one drive circuit to energize the transducer when the printing apparatus is in a printing mode, and a drive circuit to energize the transducer when it is in a measuring mode. This not only makes the printing apparatus expensive, but also, due to the increase in the number of components, less reliable.
The object of the present invention is to obviate the above-identified disadvantages. To this end, a printing apparatus has been invented wherein measurement of the electrical signal generated by the transducer in response to energization takes place when the printing apparatus is in a printing mode. There is therefore no need to interrupt the printing mode. The electrical signal is measured immediately after the transducer has been energized, the energization being such that an ink droplet is ejected with the duct operating as normal, in order to generate an image on a substrate. As a result there is no loss of productivity and in addition only one drive circuit is required for the transducer. An additional advantage is that the breakdown of the ink duct can be detected practically immediately, so that in many cases a repair operation can be carried out before any visible artefacts have appeared in an image. This means that a printing apparatus according to the present invention has a very high reliability. In one preferred embodiment the drive circuit and the measuring circuit are connected to the transducer via a common line serving as an input and output for electrical signals. This has advantages when the print-head is provided with a large number of ink ducts. The circuit can further be simplified by breaking the circuits by means of a changeover switch, so that the drive circuit is automatically opened as soon as the measuring circuit is closed. This changeover switch can be embodied by known electrical means but can also be integrated in the drive IC.
To check whether a vibration in the duct differs from a normal vibration, i.e. from a vibration when the duct is operating properly, the electrical signal generated by the transducer in response to energization can be compared with the electrical signal generated by a dummy element having the same impedance as the transducer in response to a comparable energization. Since, however, it is difficult to find a dummy element having in all circumstances exactly the same impedance as the transducer, it is preferable not to compare the electrical signal with a signal generated by a dummy element, but to characterize the electrical signal itself. For this purpose, at least one wave characteristic selected, for example, from the group comprising: amplitude, zero-axis crossing, frequency, phase and damping should be determined. It has been surprisingly discovered that in this way deviation in an ink duct can be detected with much higher accuracy. In this way it is possible to determine unambiguously what is the cause of malfunctioning of the ink duct (whether an air bubble, a solid particle clogging the duct, or a mechanical fault in the piezo element and so on) so that a repair operation can be accurately adapted to such cause.
In addition, a small deviation can be found which at that time is not yet affecting the ejection of ink droplets, for example an air bubble which is too small or still too far away from the opening of the ink duct to prevent ejection of an ink droplet. This enables preventive repair of an ink duct, so that generally there should be no artefacts appearing in an image. This is a considerable contribution to the reliability of the printing apparatus. In one preferred embodiment, a measured wave characteristic is compared with a reference value so that it is possible to determine easily whether a repair operation is required. In order further to increase the sensitivity of the measuring circuit, it can be provided with an amplifier. If an input of the amplifier is connected to the printing apparatus earth, stray capacitances (e.g. in the wiring) and leakage currents will also have hardly any effect on the measurement of the electrical signal generated by the transducer, so that the measurement accuracy further increases. In view of the simplicity of the measuring circuit in the printing apparatus according to the present invention, it is possible to provide a separate measuring circuit for all the transducers in the printing apparatus, even if there are several hundred. This makes it possible to check each duct, after an ink droplet has been ejected, for correct operation thereof, so that maximum reliability can be guaranteed.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.